The circumferential direction of a tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the rolling direction of the tire.
The transverse or axial direction of a tire is parallel to the rotation axis of the tire.
The radial direction is a direction cutting the rotation axis of the tire and perpendicular to said axis.
The rotation axis of a tire is the axis about which it rotates in normal use.
A radial or meridional plane is a plane which contains the rotation axis of the tire.
The circumferential median plane, or equatorial plane, is a plane which is perpendicular to the rotation axis of the tire and which divides the tire into two halves.
The design and the improvement of tires require their effectiveness, and especially their rolling behavior, to be able to be judged as objectively as possible. Motorcycle tires have one particular feature compared with other types of tire which is that they are used with a relatively high camber angle so as to allow cornering.
During cornering, the transverse grip of a tire is an important parameter that the tire designer wishes to be able to evaluate in order to improve its tires.
Although somewhat subjective, the current methods for evaluating the transverse grip of a tire consist of an assessment by a rider who is riding a two-wheel vehicle. Such methods allow tires to be classified with respect to one another.
Since the objective of these methods is to test the performance of tires, the rider is forced to ride the motorcycle at the grip limit, with a non-zero risk of exceeding this limit and therefore losing control of the vehicle, and possibly falling off it.